9 interrupts

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ECE473/573Microprocessor System Design, Dr. Shiue

1

Interrupt


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Interrupt Programming

An interrupt is an external or internal event that interrupts

the microcontroller to inform it that a device needs its

service.

A single 8051 can serve several devices. There are 2 ways

to do that

Interrupt: interrupt service routine (ISR) or interrupt handler

polling

8051 Several devices


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Interrupt

Whenever any device needs its service, the device notifies

the microcontroller by sending it an interrupt signal.

Upon receiving an interrupt signal, the microcontrollerinterrupts whatever it is doing and serves the devices.


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Polling

The microcontroller continuously monitors the status of a

given device; when the condition is met, it performs the

service.

After that, it moves on the monitor the next device until

everyone is serviced.

8051

Monitoring

Device 1

Device 2

:

This is NOT efficient!!


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Interrupt vs. Polling

The interrupt method is preferable because the polling

method wastes much of the microcontrollers time by

polling devices that do not need service.

For interrupt method, during the waiting, you could do

other tasks and when the TF is raised, it will interrupt the

microcontroller in what it is doing.

Monitoring by polling method

Target: JNB TF, Target

Doing waiting, you could not do anything!!


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Interrupt Service Routine (ISR)

For every interrupt, there is a fixed location in memory that

holds the address of its ISR.

The group of memory location set aside to hold theaddresses of ISRs is called Interrupt Vector Table

Interrupt Vector Table

0023 ~ 002A(8B)Serial COM interruptRI and TI

001B ~ 0022(8B)Timer 1 InterruptTF1

13 (P3.3)0013 ~ 001A (8B)External H/W Interrupt 1INT1

000B ~ 00012(8B)Timer 0 InterruptTF0

12 (P3.2)0003 ~ 000A (8B)External H/W Interrupt 0INT0

90000 ~ 0002 (3B)Reset interruptReset

PinROM locationsDescriptionInterrupt


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Reset Interrupt

There are 3 bytes of ROM spaces assigned to the reset pin.

0000

00010002

For this reason, in our program, we put LJMP as the firstinstruction and redirect the processor away from the

interrupt vector table.ORG 0

LJMPMain 3-byte instruction located to0000, 0001, and 0002

ORG 30H

Main:

Reset

0000

0001

0002

0030

0031

0032


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Interrupt Enable (IE) Register

Enabling and disabling an interrupt

EA:

EA=0 No interrupt

EA=1 Interrupt

ET2 is for Timer 2 (in 8052)

ES: enable/disable the serial port interrupt

ET1 (Timer 1 interrupt) and ET0 (Timer 0 interrupt)

EX1 (External interrupt: INT1) and EX0 (Externalinterrupt INT0)

EX0ET0EX1ET1ESET2-EAIE


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Example

Q1: Show the instructions to (a) enable the serial interrupt,

timer 0 interrupt, and external H/W interrupt 1, and (b)

disable the timer 0 interrupt, and (c) show how to disable

all the interrupt with a single instruction.


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Timer Interrupts

TF0 interrupt (for example of Timer 0)

Steps

1. IE register is enable for ET0.

2. When the timer rolls over, TF0 is raised.

3. Microcontroller is interrupted and jump to the interrupt vector table to

serve the ISR. In this way, the microcontroller can do other things until

it is notified that the timer 0 has rolled over.

1 000BH

Jump toTF0

EX0ET0

=1

EX1ET1ESET2-EA

=1

IE


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Timer Interrupts

Step 1:

ORG 0

LJMP Main

ORG 30H

Main:

Step 2:

Timer 0 000BH

Step 3: (EA=1, and ET0 =1 for Timer 0)

MOV IE, #10000010B


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Example

Q2: Write a program that continuously gets 8-bit data from

P0 and sends it to P1 while simultaneously creating a

square-wave of 200us period on Pin P2.1 (Use timer 0 to

create the square wave. Assume XTAL=11.0592MHz) Q3: Rewrite Q2 to create a square wave as below

Q4: Write a program for Timer 0 to generate a square wave

of 50Hz frequency on P1.2 (a) using polling method, and(b) using interrupt method.

1085us

15us


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External H/W Interrupt

!INT0 (0003 ~000AH) and !INT1 (0013 ~ 001AH)

Level-Triggered Interrupt

Normally high (!INT0 and !INT1) when if a low signal applied

to them, it trigger the interrupt. The low signal must be removed before RETI

!INT0 12 P3.2

!INT1 13 P3.3


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Example

Q5: Assume that the INT1 pin is connected to a switch that

is normally high whenever it goes low, it should turn on an

LED. The LED is connected to P1.3 and is normally off

when it is turned on it should stay on for a fraction of asecond. As long as the switch is pressed low, the LED

should stay ON.


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Sampling of the low level-triggered interrupt

The INTx pin must be held in a low state until the start of

the execution of ISR This duration is around 4 machine

cycle but no more.

The INTx must be brought back to high before theexecution of RETI

IT0 (TCON.0) and IT1 (TCON.2) are both 0, making

external interrupts level-triggered.


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Edge-Triggered Interrupt

SETB TCON.0 (IT0)

SETB TCON.2 (IT1)

INT1

Pulse generator

(edge-triggered)

P1.3 LED


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Example

Q6: Assume that P3.3 (INT1) is connected to a pulse

generator, write a program in which the falling edge of the

pulse will send a high to P1.3, which connected to a LED

(or buzzer). In other words, the LED is turned ON andOFF at the same rate as the pulses are applied to the INT1

pin (edge-triggered).


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Sampling of the edge-triggered interrupt

The falling edge is located by 8051 and is held by the

TCON register.

Interrupt-in-service flags

TCON.1 (IE0) hold the latch falling edge for INT0

TCON.3 (IE1) hold the latch falling edge for INT1

IE0=1 or IE1=1 in Interrupt service. They are only

cleared by RETI, soCLR TCON.1

CLR TCON.3

are NOT necessary!!!


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Serial Communication Interrupt

Remind the SCON register

TI=1 SBUF is ready to transfer the next byte.

RI=1 The received byte needs to be picked up before it

is lost.

Enable Serial comm. Interupt SETB IE.4

RITIRB8TB8RENSM2SM1SM0SCON

EX0ET0EX1ET1ES=1

ET2-EA

=1IE


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Serial Communication Interrupt

The serial interrupt is used mainly for receiving data and is

never used for sending data serially.

Phone

Call

Receiving


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Example

Q7: Write a program in which the 8051 reads data from P1and writes it to P2 continuously while giving a copy of it tothe serial com port to be transferred serially.

(XTAL=11.0592MHz, and baud rate = 9600) Q8: Write a program in which the 8051 gets data from P1and send it to P2 continuously while incoming data fromthe serial port is sent to P0. (XTAL=11.0592MHz, and

baud rate = 9600) Q9: Write a program using interrupts to do the following

(a) receive data serially and sent it to P0, (b) have P1 portread and transmitted serially, and a copy given to P2, and

(c) make timer 0 generate a square wave of 5KHzfrequency on P0.1 (XTAL=11.0592MHz, and baud rate =4800)


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Interrupt Flags

For Serial Communication Interrupt

RI (SCON.0)

TI (SCON.1)

For Timer 0 and Timer 1 interrupt TF0 (TCON.5 for Timer 0)

TF1 (TCON.7 for Timer 1)

For external HW interrupt IE0 (TCON.1 for INT0)

IE1 (TCON.3 for INT1)


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Interrupt Priority

High to Low Priority

INT0

TF0

INT1

TF1

RI + TI


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Interrupt Priority (IP) Register

IP register

PT2 (timer 2 in 8052)

PS (for serial Port)

PT1 (for timer 1)

PX1 (for external interrupt 1)

PT0 (for timer 0)

PX0 (for external interrupt 0)

PX0PT0PX1PT1PSPT2--IP


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Example

Q10: (a) program the IP register to assign the highest

priority to INT1 then (b) discuss what happen if INT0 and

TF0 are activated at the same time. Assume the interrupts

are both edge-triggered. Q11: Assume that after reset, the interrupt priority is set by

the instruction MOV IP, #00001100B. Discuss the

sequence in which the interrupts are serviced.

9 interrupts

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